The invention relates to a display screen, in particular a display screen for a cathode ray tube, comprising a phosphor composition which includes a phosphor and a phosphor coating with a group of metal-oxygen compounds.
Display screens of cathode ray tubes are provided on the inner surface with a structured display screen coating containing a phosphor composition in dot-shaped or stripe-shaped pixels. These pixels are always arranged so as to form triads. Each triad comprises a pixel with a phosphor for each one of the three primary emission colors red, blue and green. The display screen coating is customarily applied to the glass of the screen by means of a photochemical process. In the wet-chemical variant of said photochemical process, a photosensitive phosphor suspension is employed which contains the phosphor and ammonium dichromate as the photo-active component in a polyvinyl alcohol solution, which is cured by exposure using a shadow mask. In the dry variant of the photochemical process, photosensitive lacquers are employed which become sticky upon exposure. After said exposure using a shadow mask, the dry phosphor powders adhere to the sticky lacquer surfaces.
In a display screen coating, the phosphors should exhibit a high packing density, so that an optimum brightness of the display screen can be attained. In addition, the phosphor in the display screen coating should be homogeneously distributed, i.e. the photosensitive phosphor suspensions should not form reams and "pinholes" in the drying process. The individual pixels in the display screen coating should exhibit a high definition and accurately reproduce the shadow mask structure. The colors of the phosphors should retain their color purity in the coating process. In addition, the display screen coating should adhere well, and the non-exposed regions of the display screen should be haze-free.
A pre-requisite for a good display screen coating is, inter alia, a phosphor composition which can be readily processed. The phosphor composition should exhibit good sieving properties and a high dispersibility. In suspension, the phosphor composition should exhibit a stable particle size distribution, and it should be readily filterable in all pH ranges.
In order to adapt the properties of the phosphor composition to the requirements of the production process for cathode ray tubes, the phosphor powders are customarily coated with materials which should guarantee the desired technological properties of the phosphor composition.
EP-A-0 432 500 discloses a phosphor for cathode ray tubes, which comprises phosphor particles, a first layer on the surface of the phosphor particles, which layer consists of a substantially uniform SiO.sub.2 film, and a second layer on the first layer, which second layer comprises at least a metal selected from the group formed by Zn, Al and the alkaline earth metals and at least one element of the group composed of colloidal silicon oxide, aluminum oxide-sol and titanium oxide-sol having a particle size .ltoreq.50 nm.
At present, however, ever higher requirements are imposed on the quality of display screens as regards brightness, color purity, freedom from reams and definition. Consequently, also the quality requirements for phosphor compositions increase accordingly.